Recently, in connection with the molding materials, such as plastics, attempts have been made towards using the materials having a higher molecular weight or containing an increased quantity of a filler, such as an inorganic substance, with a view to improving their physical properties, such as strength, toughness or sliding properties.
However, these improved plastics exhibit only poor fluidity when residing in the die during the extrusion molding, thus leading occasionally to surface roughnesses of molded articles known as melt fracture. On the other hand, in order to prevent the die from being deformed owing to an excess pressure prevailing within the die, the molded article being then irregular in shape, one is obliged to lower the rate of extrusion, that is, to lower the yield.
For overcoming the above problem, there have been previously proposed various methods for improving the fluidity of the molding material residing within the die at the time of extrusion molding.
For example, JP Patent Laid-Open Patent Publication No. 57-51441 (1982) discloses a method wherein the molding material is fluidized while mechanical vibrations are applied to a die of an extrusion molding machine for improving the fluidity of the molding material so as to increase the rate of extrusion.
However, this known extrusion molding method suffers from the following disadvantages.
That is, since vibrations are forcedly applied to a die simply secured to the extrusion molding machine, vibrations may not be transmitted in effect, and hence no satisfactory results have been obtained.
In view of such inconvenience of the prior art method, the present inventors have previously proposed an extrusion molding method and an extrusion molding apparatus in which molding is carried out while the die is set into resonant vibrations with the aid of ultrasonic waves (JP Patent Application No. 63-294624). However, this known method has a drawback that, when a hitherto known method of adjusting the gap of the extrusion orifice necessary for producing a molded article of a uniform thickness is simply applied, the state of resonant vibrations of the die is disturbed and hence a sufficient effect proper to the resonant vibrations cannot be obtained.
The present invention provides a solution to this problem. It is an object of the present invention to provide a method for extrusion molding according to which a die having extrusion orifice gap adjustment means annexed thereto is set into resonant vibrations for improving the fluidity of the molding material in a die as well as productivity of the molded article of a uniform thickness, and an apparatus capable of carrying out the method smoothly and having an unforced constructional disposition.